For example, in the case of a painting plant a changing device for coating media, i.e. then a colour changing device, is employed when in normal operation it relatively frequently occurs that for the coating of an object a different paint is to be used than the paint with which a previous object was painted.
In the case of a colour change, the previously used paint has to be cleaned from the media-conducting ducts and lines, for which purpose a flushing agent is conveyed through the corresponding ducts and lines. In order to keep paint losses and the required quantities of flushing agent as low as possible, the so-called pigging technique is often employed, in which the coating media or the flushing agent are pushed through the ducts and lines with the aid of pigs.
In the case of changing devices and coating systems known from the market, the use of seat valves, the valve member of which is a closing element which cooperates classically with a valve seat, has become established. Such changing devices generally comprise a common flow duct which is used for all media.
Overall, in such and also in other known systems, the volume of the media-conducting ducts and lines from which a coating medium or flushing agent cannot be recovered is comparatively large. This is due, on the one hand, to the fact that many ducts and lines in such a changing device cannot be traversed by a pig. On the other hand, there are many components and parts with projections or undercuts or the like, such as line junctions or valve seats, which come into contact with paint and quite a lot of flushing agent has to be used to free all such places perfectly from paint that is present, regardless of whether the place concerned is piggable or not.